The invention relates to an electronic circuit comprising:
an input terminal and a reference terminal for receiving an input current flowing through the input terminal and the reference terminal; PA1 an output terminal for generating an output current flowing through the output terminal and the reference terminal; PA1 a first transconductance amplifier having an output coupled to the input terminal and having inputs connected to receive a voltage difference between a first node and the reference node; PA1 a second transconductance amplifier having an output coupled to the output terminal and having inputs connected to receive said voltage difference between the first node and the reference node; and PA1 means for coupling the input terminal to the first node. PA1 means for measuring the voltage difference between the first node and the reference terminal; and PA1 means for inserting said voltage difference between the input terminal and the first node.
Well-known methods of voltage-current conversion have the disadvantage of signal distortion due to circuit non-linearity, particularly at high frequencies. FIG. 1 shows a known general technique. An input voltage V.sub.S from source 2 is connected, via a series resistor 4, to an input terminal 6 and a reference terminal 8 of an electronic circuit which provides an output current i.sub.2 at an output terminal 10 in response to an input current i.sub.1 which flows into the input terminal 6. The electronic circuit comprises a first transconductance amplifier 12 with an output 14 coupled to the input terminal 6, an inverting input 16 coupled to a first node 18 and a non-inverting input 20 coupled to the reference terminal 8 which serves as signal ground. A short-circuit between the input terminal 6 and the first node 18 provides a zero signal difference between the input terminal 6 and the first node 18. The electronic circuit further comprises a second transconductance amplifier 22 with an output 25 coupled to the output terminal 10, an inverting input 24 coupled to the first node 18 and a non-inverting input 26 coupled to the reference terminal 8. The first and second transconductance amplifiers 12 and 22 have a transconductance g.sub.1 and g.sub.2, respectively. They can be very simple, e.g. single bipolar or MOS transistors, in which case the output, inverting input and non-inverting input of the transconductance amplifier corresponds to the collector, base and emitter, respectively, of a bipolar transistor or to the drain, gate and source, respectively, of a MOS transistor. Such a two transistor arrangement is known as a current mirror with a current gain i.sub.2 /i.sub.1 =g.sub.2 /g.sub.1, which in combination with the series resistor 4 provides fast voltage-current converter useful up to high frequencies.
However, conversion errors occur. It follows from the circuit diagram that: ##EQU1## V.sub.A is the signal voltage difference between the first node 18 and the reference terminal 8 and R.sub.S is the resistance of series resistor 4. In the expression for i.sub.1, the transconductance g.sub.1 is normally not well-defined, temperature dependent and nonlinear. Therefore the current i.sub.1 will suffer from distortion. The ratio g.sub.2 /g.sub.1 enables variable gain to be achieved and is generally well-defined and linear since the peculiarities of transconductance g.sub.1 are compensated by those of transconductance g.sub.2. It follows that also the output current i.sub.2 will suffer from distortion.